Android: Customizing your Android phone's ringtones and alert sounds isn't exactly difficult, but Ringtone Slicer is a lightweight audio editor that removes the need to use a bulkier tool or your desktop computer entirely. The app is great at what it does: allowing you to select songs or loops on your phone, slice them up, and then use them as custom rings or alerts.

Ringtone Slicer allows you to pick any file or folder on your SD card, browse its contents, and then load any MP3, WAV, AAC or AMR files in that folder into the app for editing. The original file isn't changed, and a modified version of it is dropped next to the original when you're finished editing. The app even supports landscape mode and pinch-to-zoom so you get more precise edits. The app is easy to use, simple, and gets the job done without a ton of overhead or unneeded add-on options.
The developer notes that the app is free and always will be free—although he may make the app ad-supported when it comes out of beta. He plans to add audio effects like fade in/out and volume adjustment to the editor in upcoming releases, but even as it is now, Ringtone Slicer is great for alerts, but it's also a pretty handy on-the-go audio editor for Android.